Just the UI

Lots of people use React as the V in MVC.
Since React makes no assumptions about the rest of your technology stack,
it's easy to try it out on a small feature in an existing project.

Virtual DOM

React abstracts away the DOM from you, giving a simpler programming model and better performance. React can also render on the server using Node, and it can power native apps using React Native.

Data flow

React implements one-way reactive data flow which reduces boilerplate and is
easier to reason about than traditional data binding.

A Simple Component

React components implement a render() method that takes input data and
returns what to display. This example uses an XML-like syntax called
JSX. Input data that is passed into the component can be accessed by
render() via this.props.

JSX is optional and not required to use React. Try
clicking on "Compiled JS" to see the raw JavaScript code produced by
the JSX compiler.

A Stateful Component

In addition to taking input data (accessed via this.props), a
component can maintain internal state data (accessed via this.state).
When a component's state data changes, the rendered markup will be
updated by re-invoking render().

An Application

Using props and state, we can put together a small Todo application.
This example uses state to track the current list of items as well as
the text that the user has entered. Although event handlers appear to be
rendered inline, they will be collected and implemented using event
delegation.

A Component Using External Plugins

React is flexible and provides hooks that allow you to interface with
other libraries and frameworks. This example uses marked, an external
Markdown library, to convert the textarea's value in real-time.